Processing of sound is a collective activity of neuronal circuits that begins with the transmission of acoustic input from the cochlea to the brain. Auditory nerve fibers activate neurons in the cochlear nuclei, which then signal synaptically to other neurons in the cochlear nuclei as well as to higher centers. The dorsal cochlear nucleus (DCN) has long been considered a primary auditory nucleus because it receives input directly from auditory nerve fibers, but T stellate cells of the ventral cochlear nucleus (VCN) also project with excitatory synapses to the DCN. What is the relationship between these two sources of acoustic input to the DCN? Are the same neurons targeted by auditory nerve fibers and T stellate cells? Do T stellate cells in the VCN that are activated by a particular sound target cells in the DCN that are activated by the same sound? More generally, does the connectivity within the auditory brain stem reflect a cell-type based code or a more subtle functional code? Although circuitry can be studied by existing anatomical and functional techniques at the level of connections between different cell types, classical techniques cannot readily address the interactions that occur within populations of neurons that cooperate during the performance of specific tasks or sensory responses. The present application introduces a novel experimental design that will target neurons linked by their participation in behaviors under defined experimental conditions. Newly developed mouse models will use immediate-early gene expression to target a genetically-encoded voltage sensor to electrically active neurons. The voltage probe, a hybrid voltage sensor (hVOS), can detect subthreshold synaptic potentials in a single cell in a brain slice in a single trial without averaging. Immediate-early genes will be activated by presenting animals with auditory stimulation in the form of pure tones or white noise. This will drive hVOS probe expression selectively in the neurons that respond to auditory stimulation. Slices from the cochlear nuclei will then be used in voltage imaging experiments to study the circuit relations within these unique sets of labeled neurons. This approach will thus reveal the synaptic connections between selected populations of neurons linked by participation in responses to particular sounds. Pure tones will drive hVOS probe expression in isofrequency laminae in the VCN and DCN. hVOS imaging will then reveal the interactions between functionally related, labeled T stellate cells within these isofrequency bands in the VCN, as well as between T stellate cells and their targets in the DCN. We will then turn to white noise sound to target neurons that respond to broadband stimuli and assess their functional connections. We will use both pure tones and noise to label cells and investigate interactions between T stellate cells and their targets to learn whether T stellate cells are major sources of acoustic input. These experiments will reveal new features of the circuitry engaged in specific forms of auditory processing. We will determine if neurons that respond to specific sounds form unique networks dedicated to the sensory inputs that activate them.